Fine-scale foraging behavior reveals differences in the functional roles of herbivorous reef fishes

Abstract

Efforts to understand and protect ecosystem functioning have put considerable emphasis on classifying species according to the functions they perform. However, coarse classifications based on diet or feeding mode often oversimplify species' contributions to ecological processes. Behavioral variation among superficially similar species is easily missed but could indicate important differences in competitive interactions and the spatial scale at which species deliver their functions. To test the extent to which behavior can vary within existing functional classifications, we investigate the diversity of foraging movements in three herbivorous coral reef fishes across two functional groups. We find significant variation in foraging movements and spatial scales of operation between species, both within and across existing functional groups. Specifically, we show that movements and space use range from low frequency foraging bouts separated by short distances and tight turns across a small area, to high frequency, far-ranging forays separated by wide sweeping turns. Overall, we add to the burgeoning evidence that nuanced behavioral differences can underpin considerable complementarity within existing functional classifications, and that species assemblages may be considerably less redundant than previously thought.

Keywords: complementarity; coral reefs; foraging behavior; functional traits; movement.

FIGURE 1

Photo of two Siganus corallinus individuals (credit: Victor Huertas)

FIGURE 2

Foraging paths and resulting size and distribution of short‐term feeding areas (direct observation). (a–c) Example foraging paths for all three species. Green = the parrotfish Sc. schlegeli, yellow and blue = the rabbitfishes, Si. Vulpinus, and Si. corallinus, respectively. Dots represent foraging locations, while lines represent vectors between foraging events. Path insets not scaled by area, but relative size can be seen in the wider figure. (d) Distribution of feeding areas (MCP) for each species on Big Vicki's Reef with inset showing location of Big Vicki's Reef on Lizard Island, colors as above

FIGURE 3

Relative size of daily foraging areas (acoustic telemetry). (a) Spatial distribution of daily foraging areas on Watson's Reef. Dotted and dashed lines mark the 95% Kernel Utilization Distributions (KUDs), while filled, transparent areas mark the 50% core areas. Fish numbers are given for all paired and the two singular individuals. Colors as above. (b) Location of Watson's Reef on Lizard Island

FIGURE 4

Differences in the six metrics used to evaluate foraging paths of the three species. Asterisks indicate significant differences among species via ANOVA. Si. vulpinus foraging movements are characterized by: large areas, wide turns, higher speeds, and longer interforay distances; Si. corallinus foraging movements are characterized by: small areas, sharp turns, low feeding frequency, low speed, and short interforay distances; Sc. schlegeli occupy intermediate positions but display the highest frequency of foraging. Boxplots represent the median and interquartile range of each foraging trait. Dashed lines separate the two grazing rabbitfishes from the scraping parrotfish

FIGURE 5

Nonmetric multidimensional scaling ordination depicting differences in foraging paths of three species: the rabbitfishes Siganus vulpinus (yellow), Si. corallinus (blue), and the parrotfish Scarus schlegeli (green). Convex hulls represent minimum convex polygons for all individuals of a species. Vectors represent the loadings

FIGURE 6

Feeding rate, movement rate, and feeding efficiency of the three species. Asterisks indicate significant differences among species via ANOVA. Boxplots represent the median and interquartile range of each foraging trait. Dashed lines separate the two grazing rabbitfishes from the scraping parrotfish